Plasma-based light sources, such as laser-produced plasma (LPP) sources and discharge-produced plasma (DPP) sources, are often used to generate soft X-ray, extreme ultraviolet (EUV), and/or vacuum ultraviolet (VUV) light for applications such as defect inspection, photolithography, or metrology. In overview, in these plasma light sources, light having the desired wavelength is emitted by plasma formed from a target material having an appropriate line-emitting or band-emitting element, such as Xenon, Tin, Lithium or others. For example, in an LPP source, a target material is irradiated by an excitation source, such as a laser beam, to produce plasma, and in a DPP system, a target material is excited by an electrical discharge, for example using electrodes, to produce plasma.
For these sources, the light emanating from the plasma is often collected via a reflective optic, such as a collector optic (e.g. a near-normal incidence or grazing incidence mirror). The collector optic directs, and in some cases focuses, the collected light along an optical path to an intermediate location where the light is then used by a downstream tool, such as a lithography tool (i.e. stepper/scanner), a metrology tool or a mask/pellicle inspection tool.
During operation of the plasma-based illumination system, debris such as target material gas, atomic vapor, high energy ions, neutrals, micro-particles, and/or contaminants (e.g. hydrocarbons or organics) may be emitted from various sources including, but not limited to, the target material, plasma site, plasma-facing components, eroded surfaces in proximity of the target material or the plasma, a target-forming structure, and/or any other component within a plasma-based light source. These debris can sometimes reach the reflective optic, or other components, such as a laser input window or diagnostic filters/detectors/optics, and degrade their performance and/or cause irreparable damage. In addition, high energy ions, neutrals and other micro-particles emitted by the plasma can erode/sputter light source components creating further debris that can interfere with the efficient operation of the light source.
In addition to damaging light source components, the plasma generated debris, and especially the gas/atomic vapor can undesirably attenuate light emitted by the plasma. For example, for an EUV source, where Xenon is used as a target material, the introduction of a buffer gas (e.g. for ion stopping or light source temperature control) may lead to significant losses in EUV transmission due to Xenon gas, which strongly absorbs EUV light, and mixes with the buffer gas. In more quantitative terms, the light transmission of 13.5 nm EUV light through 1 Torr*cm (pressure*distance) of Xenon gas at room temperature is ˜44%, while the light transmission of 13.5 nm EUV light through 1 Torr*cm of Argon is ˜96%.
The use of coils has been suggested to protect a reflective optic by deflecting charged particles using magnetic fields. However, coils producing magnetic fields require significant design complexity, are expensive, and are only capable of deflecting ions. Thus, the use of magnetic fields is ineffective in stopping neutrals (and neutral particles), which are often produced when ions undergo charge exchange with a buffer gas.
With the above in mind, Applicants disclose a Plasma-based Light Source and corresponding methods of use.